1. Field of the Invention
This invention relates to an apparatus for examining the response of an organism to changes in its environment. This invention particularly pertains to improvements in cage-culture turbidostat design and operation for culturing organisms and cells at a constant population density.
2. Description of Related Art
A cage-culture turbidostat is a device useful, for example, for examining both the chronic toxicity and the acute toxicity of various substances to a particular microorganism or cell. In a cage-culture turbidostat a colony of microorganisms or cells is grown in an isolated vessel or growth chamber as a suspension culture at a constant population density. The culture is kept in suspension in the growth chamber by a mechanical or magnetic stirrer. The turbidity of the culture suspension normally is used as an indication of the cellular population. A photo-sensitive detector measures the magnitude of a signal transmitted through a representative portion of the culture vessel from a light source. In the case where light must be provided for the growth of the organisms, the same light source can also be used to monitor and regulate turbidity. When the strength of the received signals falls below some pre-set value, indicating an increase in the cell density in the culture vessel, a control circuit is energized and a portion of the biomass is removed from the culture vessel, for example, by simply opening a drain valve or by pumping. As soon as such harvesting has lowered the cell population in the culture vessel to a point where the strength of the received light signal increases above a second pre-set value, the control circuit terminates the remedial harvesting action.
Thus, by measuring variations in the turbidity of the culture suspension, using one or more photo-sensitive detectors, the quantity of growing cells in the culture vessel can be automatically controlled. The growth rate of the cells or microorganisms under investigation then can readily be determined from the operating time of the harvesting or dilution system, which normally is integrated over some convenient time period.
In the standard design, the growth chamber of the culture vessel is isolated or confined by spaced screens, filters or porous membranes, hereinafter referred to as filters. The particular filters used depend, inter alia, upon the size of the cell or organism being studied. The filters are designed to allow substantially unrestricted passage of fresh and spent nutrient medium into and out of the growth chamber respectively, but to prevent any loss of the cultured cells or organisms and the ingress of unwanted cells and organisms. Such design allows one to independently vary the flow rate of the nutrient medium and the density of cells in the growth chamber while avoiding the problem of wash-out often encountered in chemostats. Thus, this device provides a powerful analytical tool for physiological and nutritional studies including, for example, pollution monitoring.
A common and debilitating problem with this standard design is the accumulation of the cultured organism or cell on the internal filter walls of the growth chamber. This accumulation of biomass interferes with uniform inflow and outflow of the nutrient medium. To prevent such accumulation and the clogging of the growth chamber which inevitably results, the prior art has used rotating filters and intense agitation near the filter membranes. In a more recent approach to keeping the membranes clean, described in Skipnes et al., (1980) Appl. Environ. Microbiol. 40:3180325, a reversible nutrient supply pump is used to periodically reverse the direction of nutrient flow through the growth chamber of the turbidostat.
The former solutions to this problem involve complicated designs and may cause damage to the organisms or cells being cultivated thereby altering the results obtained. The latter approach undesirably introduces a significant fraction of spent nutrient medium back into the culture vessel thereby altering the basic nature of the culture process and significantly complicating the analysis of the overall results obtained.
It is an object of the present invention to provide an improved apparatus for examining the population dynamics of a colony of cells or microorganisms as a function of nutrient medium composition.
It is another object of the present invention to provide an improved cage-culture turbidostat design which substantially prevents the accumulation of microorganisms or cells on the filter membrane of the growth chamber.
It is yet another object of the present invention to provide a cage-culture turbidostat design and method of operation which avoids the problems introduced by the prior art solutions to the membrane clogging problem.